Thin-film transistor-driven linear image sensors are well-known. Reference is made to the following publications for descriptions of particular architectures and addressing circuitry and methods of fabrication: G. Brunst, et. al., MAT. RES. SOC. SYMP. PROC., Vol. 118, pg. 249 (1988); H. Ito, et. al., MAT. RES. SOC. SYMP. PROC., Vol. 95, pg. 437 (1987). These papers, whose contents are incorporated by reference, describe devices containing, for example, 300 dots per inch sensors and in linear form extending over, say, a standard page size of 81/2 inches resulting in a large number of individual pixel elements, in particular, about 2500 in a black and white system. If it is desired to increase the sensor density to 400 per inch and cover the long dimension of a standard 81/2.times.11 inch page, then the total increases to about 4800 pixels. To expand the system to include color, implying in a 3-color system a primary color of one red, one green and one blue sub-pixel jointly representing one pixel element, then the array size increases to nearly 15,000 individual sub-pixel sensor elements.
In a typical system that uses photodiodes as the sensors, readout is accomplished by transferring the photo-charge from the photodiodes to a capacitor on which a voltage is developed which is proportional to the signal charge. The problem then arises as to how to address for readout these large numbers of elements in the simplest possible way with a minimum of interconnect lines. Moreover, in a color system, where the incident light is filtered through a color filter, the reduced light level incident on the sensor implies a need for increased responsivity of the sensors, where responsivity is the measured voltage per unit exposure. For the typical voltage sensing photodiode element, increased responsivity can be obtained without increasing size by reducing the capacitance which is used to develop the sense voltage. This capacitance is often the data line capacitances, and a low value is best achieved with reduced line lengths, and fewer crossovers.
These goals cannot be met using conventional matrix addressed array architectures. The most common approach of multiplexing data lines in a data line crossover matrix has several problems. The first problem is the large number of data lines required in order to handle the high data rate needed for color scanning. This leads to excessive array size and unacceptably high crosstalk between the data lines. A modification of this structure using ground shields between each data line crosspoint minimizes crosstalk but at the expense of extremely high data line capacitance and therefore low responsivity.
Another architecture commonly employed in such matrix-addressed scan arrays is a meander architecture where data lines pass back and forth between the photodiodes to avoid any crossing of the data lines. The problem with this approach for color arrays is that the lines must be extremely narrow to pass between the sensors and they must cover larger distances, adversely impacting yield and rendering the data line capacitances too large to achieve high responsivity.